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Re: USB Audio Interface

Posted: Thu May 10, 2018 5:17 pm
by IngoQ
I did a little research on this, but I am still not entirely sure what to do.

I have three hifi components, all class 2 and double insulated. Which in fact means, they have a more or less floating signal ground and no reference ground. When these get connected together, they will equalise their potential via the audio cable. Since this would degrade the quality of the actual signal by inducting noise in the inner core, it is advisable to establish an additional common ground. This is called a Parallel Earth Conductor or PEC.

So far I can follow.

If I now connect this setup to a device that is class 1 and therefore earthed, again the potentials try to equalise and in the worst case fry the weaker component, i.e. the USB port or said poor Falcon...

So I could of course connect my signal ground to earth, so PC and Hifi have the same potential. But I am not sure, this is a good idea. If there is enough current to fry a USB port, it will just run through my earth wire and probably damage the involved power supplies.

Normally one would use a ground loop isolator for either the audio signal or the usb signal of the audio interface. Having it in the audio path will probably more practical, since it would prevent me from destroying my precious machines... albeit being cheaper :)

So is this the way to go? Leaving the hifi components with their flowing, but common signal ground and isolating everything I connect with it? Or is having a common ground with the earthed PC safe and the better way?

Re: USB Audio Interface

Posted: Thu May 10, 2018 6:08 pm
by exxos
Well, rule 1 is never connect equipment together with the power on.. That's how I fried my Falcon :(.. As a floating earth will come into contact with a gnd ref earth and you end up with a voltage spike which kills stuff. Once all the gnds connected, things are generally happy.

If you use good solid audio cables, then each equipment will find its own gnd somewhere.. though the problem is with gnd is you can end up with "ground loops" which can actually cause more hum in some cases. So some trial and error is needed to see which works best for you.

The other method is using differential inputs, where common noise between equipment gets canceled out. Normally what XLR connectors use. Though with decent "thick" audio cables, there shouldn't be any need for it. The stuff which has huge outer braided sheiling (like coax has) then a tin foil wrap as well. Noise happens due to voltage drops across the gnd part of the cables, so the more copper you have there the better.

I would try good cables first, then you can try gnd wires to various other equipment and see if it helps or makes things worse.. Though to gnd equipment properly, you need to be thinking more copper straps for earthing wires. Resistance to gnd needs to be less than the resistance of the gnd part of the audio cables else it won't do anything. Though then you're basically getting into professional racks with bus bars.. might be bit overkill :)

Re: USB Audio Interface

Posted: Thu May 10, 2018 7:46 pm
by rpineau
Or use fiber connections (TosLink) :) .. I know it's not always possible, but hey, no ground here. We use this in the network industry to connect building as their ground potential is never the same and if you were to run CAT5/6/7 between 2 building you're pretty much sure to burn something :). Fiber is your friend.
2nd best, as Chris said, differential connection.

Re: USB Audio Interface

Posted: Fri May 11, 2018 10:37 am
by IngoQ
Yes, I daily work with fibre wiring as well. But we're talking vintage audio here, so no toslink and no symetric signal path either ;-)

I'll try my luck with providing a thick, common ground to all components, which is quite easy to do. Good quality cinch cables are of course in the picture as well. And then see, whether the audio interface already is isolated or if I need a ground loop isolator. I measured around 50V between amp and PC chassis, but hardly any current flow. So maybe this not a big issue.